Just my thoughts but I don't think your drinking enough water.
General guideline is to drink half your body weight in ounces daily. If we use 8 glasses as an example, that's roughly 64oz of water. Barely enough for a 125-130lb person. I'm assuming you weigh more than that since your a guy. Keep in mind this is the range without any type of bowel cleansing.
Secondly how much dietary fiber are you getting a day? Sounds like your not eating many leafy greens, fruits are great, but without the fiber, your going to get backed up.
I've also heard its normal to feel constipated for the first few days on P&B shakes.
I just started P&B's a couple days ago and dramatically increased my water intake. Went from 70-80oz a day to nearly a gallon and have been having 3-4 small bowel movements a day on one shake.
Try to aim for 30-40 grams of fiber through food and dramatically increase your water intake.